ZDNS

ZDNS is a command-line utility that provides high-speed DNS lookups. ZDNS is written in Go and contains its own recursive resolution code and a cache optimized for performing lookups of a diverse set of names. We use https://github.com/miekg/dns to construct and parse raw DNS packets.

As an example, the following will perform MX lookups and a secondary A lookup for the IPs of MX servers for the domains in the Alexa Top Million:

cat top-1m.csv | ./zdns MX --ipv4-lookup --alexa

Install

ZDNS can be installed by checking out the repository and running go build.

git clone https://github.com/zmap/zdns.git
cd zdns/zdns
go build

You cannot just run go get because we use a forked version of miekg's DNS library that has additional performance fixes.

Usage

ZDNS provides several types of modules:

• Raw DNS modules provide the raw DNS reponse from the server similar to dig, but in JSON. There is a module for (nearly) every type of DNS record
• Lookup modules provide more helpful responses when multiple queries are required (e.g., completing additional A lookup if a CNAME is received)
• Misc modules provide other additional means of querying servers (e.g., bind.version)

We detail the modules below:

Raw DNS Modules

The A, AAAA, AFSDB, ANY, ATMA, AVC, AXFR, BINDVERSION, CAA, CDNSKEY, CDS, CERT, CNAME, CSYNC, DHCID, DMARC, DNSKEY, DS, EID, EUI48, EUI64, GID, GPOS, HINFO, HIP, HTTPS, ISDN, KEY, KX, L32, L64, LOC, LP, MB, MD, MF, MG, MR, MX, NAPTR, NID, NINFO, NS, NSAPPTR, NSEC, NSEC3, NSEC3PARAM, NSLOOKUP, NULL, NXT, OPENPGPKEY, PTR, PX, RP, RRSIG, RT, SVCBS, MIMEA, SOA, SPF, SRV, SSHFP, TALINK, TKEY, TLSA, TXT, UID, UINFO, UNSPEC, and URI modules provide the raw DNS response in JSON form, similar to dig.

For example, the command:

echo "censys.io" | ./zdns A

returns:

{
  "name": "censys.io",
  "class": "IN",
  "status": "NOERROR",
  "data": {
    "answers": [
      {
        "ttl": 300,
        "type": "A",
        "class": "IN",
        "name": "censys.io",
        "data": "216.239.38.21"
      }
    ],
    "additionals": [
      {
        "ttl": 34563,
        "type": "A",
        "class": "IN",
        "name": "ns-cloud-e1.googledomains.com",
        "data": "216.239.32.110"
      },
    ],
    "authorities": [
      {
        "ttl": 53110,
        "type": "NS",
        "class": "IN",
        "name": "censys.io",
        "data": "ns-cloud-e1.googledomains.com."
      },
    ],
    "protocol": "udp",
    "resolver": "30.128.52.190:53"
  }
}

Lookup Modules

Raw DNS responses frequently do not provide the data you want. For example, an MX response may not include the associated A records in the additionals section requiring an additional lookup. To address this gap and provide a friendlier interface, we also provide several lookup modules: alookup and mxlookup.

mxlookup will additionally do an A lookup for the IP addresses that correspond with an exchange record. alookup acts similar to nslookup and will follow CNAME records.

For example,

echo "censys.io" | ./zdns mxlookup --ipv4-lookup

returns:

{
  "name": "censys.io",
  "status": "NOERROR",
  "data": {
    "exchanges": [
      {
        "name": "aspmx.l.google.com",
        "type": "MX",
        "class": "IN",
        "preference": 1,
        "ipv4_addresses": [
          "74.125.28.26"
        ],
        "ttl": 288
      },
      {
        "name": "alt1.aspmx.l.google.com",
        "type": "MX",
        "class": "IN",
        "preference": 5,
        "ipv4_addresses": [
          "64.233.182.26"
        ],
        "ttl": 288
      }
    ]
  }
}

Other DNS Modules

ZDNS also supports special "debug" DNS queries. Modules include: BINDVERSION.

Local Recursion

ZDNS can either operate against a recursive resolver (e.g., an organizational DNS server) [default behavior] or can perform its own recursion internally. If you are performing a small number of lookups (i.e., millions) and using a less than 10,000 go routines, it is typically fastest to use one of the common recursive resolvers like Cloudflare or Google. Cloudflare is nearly always faster than Google. This is particularly true if you're looking up popular names because they're cached and can be answered in a single round trip. Otherwise, performing iteration internally is much faster, because you can run with tens of thousands of concurrent threads without DOS'ing and/or rate limiting your recursive resolver.

To perform local recursion, run zdns with the --iterative flag. When this flag is used, ZDNS will round-robin between the published root servers (e.g., 198.41.0.4). In iterative mode, you can control the size of the local cache by specifying --cache-size and the timeout for individual iterations by setting --iteration-timeout. The --timeout flag controls the timeout of the entire resolution for a given input (i.e., the sum of all iterative steps).

Output Verbosity

DNS includes a lot of extraneous data that is not always useful. There are four result verbosity levels: short, normal (default), long, and trace:

• short: Short is the most terse result output. It contains only information about the responses
• normal: Normal provides everything included in short as well as data about the responding server
• long: Long outputs everything the server included in the DNS packet, including flags.
• trace: Trace outputs everything from every step of the recursion process

Users can also include specific additional fields using the --include-fields flag and specifying a list of fields, e.g., --include-fields=flags,resolver. Additional fields are: class, protocol, ttl, resolver, flags.

Name Server Mode

By default ZDNS expects to receive a list of names to lookup on a small number of name servers. For example:

echo "google.com" | ./zdns A --name-servers=8.8.8.8,8.8.4.4

However, there are times where you instead want to lookup the same name across a large number of servers. This can be accomplished using name server mode. For example:

echo "8.8.8.8" | ./zdns A --name-server-mode --override-name="google.com"

Here, every line piped in ZDNS is sent an A query for google.com. ZDNS also supports mixing and matching both modes by piping in a comma-delimited list of name,nameServer. For example:

echo "google.com,8.8.8.8" | ./zdns A will send an A query for google.com to 8.8.8.8 regardless of what name servers are specified by --name-servers= flag. Lines that do not explicitly specify a name server will use the servers specified by the OS or --name-servers flag as would normally happen.

Running ZDNS

By default, ZDNS will operate with 1,000 light-weight go routines. If you're not careful, this will overwhelm many upstream DNS providers. We suggest that users coordinate with local network administrators before performing any scans. You can control the number of concurrent connections with the --threads and --go-processes command line arguments. Alternate name servers can be specified with --name-servers. ZDNS will rotate through these servers when making requests. We have successfully run ZDNS with tens of thousands of light-weight routines.

Unsupported Types

If zdns encounters a record type it does not support it will generate an output record with the type field set correctly and a representation of the underlying data structure in the unparsed_rr field. Do not rely on the presence or structure of this field. This field (and its existence) may change at any time as we expand support for additional record types. If you find yourself using this field, please consider submitting a pull-request adding parser support.

License

ZDNS Copyright 2020 Regents of the University of Michigan

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See LICENSE for the specific language governing permissions and limitations under the License.